The Imaging Core has two purposes: (1) to perform acquisition, reconstruction, processing and archiving of the PET, MR and CT data for Projects 1,2 and 3. and (2) to validate recently developed MR methods of cerebral blood flow (CBF) and oxygen extraction fraction (OEF) estimation against gold-standard techniques. All PET studies will be performed on the Siemens 961 ECAT EXACT HR 47 PET scanner located in the Neurocritical Care PET Research Facility in the Neurology Neurosurgery Intensive Care Unit. This facility is dedicated to scientific research with studies of acute brain injury given top priority. It is outfitted with all the life support equipment available elsewhere in the NNICU. Clinical CT and MR studies from projects 1,2 and 3 will be performed on clinical equipment at Barnes-Jewish Hospital. Barnes-Jewish Hospital and Washington University School of Medicine have a long and successful collaborative arrangement for clinical imaging research. Currently, a formal agreement exists between Barnes-Jewish Hospital and the Mallinckrodt Institute of Radiology to use the hospital's clinical equipment for research purposes. After approval by the Research Committee, investigators are authorized to use the facilities as necessary to carry out research including access at short notice to study patients with acute disease. Image files are easily transferred through the hospital's electronic radiology system directly to the computer processing facilities in the East Building. We have successfully used this system over the past 6 years to study patients with acute ischemic stroke, intracerebral hemorrhage and traumatic brain injury. The clinical CT and MR studies described in this proposal have been approved through this mechanism. Validation of MR methods will be carried out in non-human primates in the East Building Imaging Research Facility by performing the following specific aims: (1) To develop a software package for obtaining quantitative estimates of MR CBF, OEF, and CMRO2 (2)To validate MR-measured CBF against a Kety-Schmidt technique under a range of experimental conditions in non-human primates (3)To validate MR-measured whole brain OEF against direct measurements of cerebral arteriovenous oxygenation difference (A-VDO2) in non-human primates. These validation studies will provide the basis for future studies of human pathophysiology.